High intensity sodium vapor lamps of the present kind are described in U.S. Pat. No. 3,248,590, Schmidt, entitled "High Pressure Sodium Vapor Lamp." These lamps utilize a slender tubular envelope of light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises sodium along with a rare gas to facilitate starting, and mercury for improved efficiency. The ends of the alumina tube are sealed by suitable closure members affording connection to thermionic electrodes which may comprise refractory metal structures activated by electron emissive material. The ceramic arc tube is generally supported within an outer glass envelope or jacket provided at one end with the usual screw base. the electrodes of the arc tube are connected to the terminals of the base, that is to shell and center contact, and the interenvelope space is usually evacuated in order to conserve heat.
For maximum efficiency in high pressure sodium vapor lamps xenon is used as the starting gas. The use of xenon provides an advantage in efficiency of 10% or more over the lighter inert gas neon, and even more over a Penning mixture, such as a 99% neon-1% argon mixture. However the choice of xenon makes the lamp difficult to start. The starting voltage requirement has commonly been met by including in the ballast an electronic circuit which serves as a source of short duration high voltage pulses. After the lamp is ignited, the voltage across it is reduced and a sensing circuit responds thereto and disables the starting pulse generator. Such circuitry is relatively expensive and is proportionately more so in the smaller sizes of lamps.
Another expedient which has been used for starting is a thermal switch such as a bi-metal which is connected in series with a heating coil across the arc tube terminals. At starting the lamp is short circuited by the switch and current flows through the heater. This heats the switch until it opens whereupon interruption of current flow through the ballast produces an inductive voltage surge which should start the lamp. The switch and its heater are designed to have enough thermal inertia to remain deflected until the arc tube warms up and heats it by radiation. Should the switch contacts open at a moment in the A.C. cycle when the induced voltage is insufficient to start the lamp, an interval will follow during which the bi-metal must cool, close, and be heated to reopen again. This cycle may take well in excess of a minute; in a typical commercially available lamp manufactured in England, the cycle took from 11/2 to 2 minutes and the lamp frequently failed to start after 3 such cycles. The poor reliability of the lamp with respect to its starting characteristics, frequently entailing delays of 5 minutes or more before the arc tube even starts to warm up, has prevented general acceptance of this design.